Page:Popular Science Monthly Volume 51.djvu/88

80 or other rock existing as a ledge. Owing to the loss of the element of elasticity, the road would soon become weakened in its coherency, and the rate of wear would be much increased. It is therefore customary to excavate, when a cut in rock is necessary, some four inches below subgrade, and to fill in to subgrade with gravel on which the broken stone is placed as before.

Another modification is practiced when clayey, wet ground is encountered. Under these circumstances it is generally best to excavate some sixteen inches below the finished grade and spread on a layer of gravel four inches deep. Upon this Telford foundation is laid by hand to a depth of eight or ten inches and carefully rolled (Plate III). A layer of broken stone is then put on, and a finishing coating of screenings is added as before.

As to the character of the roads already constructed in Massachusetts, Prof. N. S. Shaler, of the Highway Commission, informed the writer that, in his opinion, they are in no way inferior, in so far as quality and durability are concerned, to the celebrated Swiss roads.

So well pleased are the people of Massachusetts with the State roads already constructed, and so active are they in the cause of good roads, that the Legislature appropriated the additional sum of four hundred thousand dollars for highway construction during the year 1895 and five hundred thousand dollars in 1896.

At first glance it would seem that the engineering skill necessary to construct a Macadam road would not be of a particularly high order; and yet the problems involved in building roads in the latitude of Massachusetts, where great variations exist in the character of the soil, owing to the glacial conditions that once existed here, call for engineering ability of a peculiar kind, as well as an extended experience in the treatment of special cases and the economical application of the materials at hand.

As an adjunct of the Highway Commission, a laboratory has been established in Cambridge, where the systematic study of road materials is carried on. It has come to be generally recognized that materials which possess the necessary qualities for a good road stone are both limited in kind and in quantity. It is the object of these laboratory investigations to classify the road stones of the State in the order of their fitness for this purpose, and to prepare a map showing the area and location of the most desirable varieties. Here are investigated the questions of the rate of wear of stones under impact, and the cementing and recementing value of the powdered rock on which the life of the road depends in a large measure. The hardness and toughness also come within the scope of the experimental work. Experiment has shown that a stone must possess certain all-around properties in order to come up to the desired standard. For